In an environment in which plural external storage devices having different function control interfaces are intermixed, when a function of a storage device is controlled from a computer, a common interface for controlling the function of the storage device is provided. A device that provides the common interface manages an interrelationship between a storage area recognized by a host computer and a storage area provided by the storage device and associates a storage area which becomes a target of a function control instruction with the storage device that provides the storage area. A type of the storage device that provides the storage area which becomes the target of the function control instruction is identified and function control is ordered through a function control interface unique to the device.